1. Field of the Invention
The present invention relates to an air-fuel ratio detecting device known as a linear A/F (air-fuel ratio)sensor for detecting an air-fuel ratio, and more particularly to an air-fuel ratio detecting device for accurately detecting a stoichiometric air-fuel ratio for an air-fuel mixture to be supplied to a combustion apparatus such as an internal combustion engine.
2. Related Art
There has been proposed a linear A/F sensor utilizing the oxygen concentration cell capability and oxygen ion pumping capability of zirconia, for detecting whether the air-fuel ratio is on a leaner or richer side of a stoichiometric ratio and also for detecting the value of the air-fuel ratio (see Japanese Laid-Open Patent Publication No. 63(1988)-36140).
One conventional linear A/F sensor will be described below with reference to FIGS. 13 through 16 of the accompanying drawings.
FIG. 13 shows a linear A/F sensor including a sensor cell 20 and a pump cell 21 which are shown detached from each other, and each include a stabilized zirconia device. The sensor cell 20 and the pump cell 21 are coupled to each other through an insulation layer 22. The sensor cell 20 and the pump cell 21 have respective diffusion holes 23, 24 defined therein for passing therethrough exhaust gases while controlling the speed thereof. The insulation layer 22 has a detecting cavity 25 defined therein into which exhaust gases can be introduced through the diffusion holes 23, 24 by the sensor cell 20 and the pump cell 21. The detecting cavity 25 serves as an element for controlling the speed at which the exhaust gases are diffused. The insulation layer 22 also has a reference chamber 25a positioned below the detecting cavity 25 in spaced-apart relation thereto, the reference chamber 25a being defined between the sensor cell 20 and the pump cell 21. A reference gas such as atmospheric air is introduced into the reference chamber 25a through a communication hole (not shown).
As shown in FIG. 14, the sensor cell 20 has porous electrodes 26, 27 of platinum, and the pump cell 21 has porous electrodes 28, 29 of platinum. The linear A/F sensor also has an electric heater 30 for heating itself to a temperature range, e.g., 800.degree..+-.100.degree. C., in which the sensor cell 20 and the pump cell 21 can operate without fail.
The sensor cell 20 functions as a conventional O.sub.2 sensor for developing an electromotive force, depending on the oxygen concentration difference, between the electrodes 26, 27. The pump cell 21 serves to pump oxygen from a negative electrode to a positive electrode when an electric current (pump current Ip) is caused to flow between the electrodes 28, 29.
A controller 31 detects an electromotive force Vs developed by the sensor cell 20, and also controls the pump current Ip through a feedback loop to energize the pump cell 21 in order to keep an oxygen concentration corresponding to a stoichiometric ratio in the cavity 25 or the diffusion holes 23, 24. Since the pump current Ip continuously varies with respect to the air-fuel ratio, as shown in FIG. 15, the air-fuel ratio can be calculated from the pump current Ip.
More specifically, the controller 31 includes a comparator 1 and an integrator amplifier 2 with positive and negative power supplies. The comparator 1 compares the electromotive force Vs and a reference voltage Vref corresponding to the stoichiometric ratio. The output signal from the comparator 1 is integrated by the integrator amplifier 2, whose integral output signal is applied as the pump current Ip to the pump cell 21 through a resistor 5.
At this time, a voltage drop across the resistor 5 is detected by a current detector 3 which produces a voltage signal commensurate with the pump current Ip. Therefore, the pump current Ip is detected indirectly by the current detector 3. The output signal of the current detector 3 is applied to a step-up circuit 4 which then produces an output signal Vout, in the range of from 0 to 5 volts, as representing the air-fuel ratio, according to the following equation: EQU Vout=G.Ip+Vstp (1)
where G is the current-to-voltage conversion gain of a current-to-voltage converter which is composed of the resistor 5 and the current detector 3, and Vstp is a step-up voltage in the range of from 0 to 5 volts.
With the conventional linear A/F sensor, the air-fuel ratio is detected depending on the pump current Ip which is produced under the feedback control. Therefore, the detected air-fuel ratio is subjected to errors of the feedback control circuit arrangement, such as fluctuation of the reference voltage Vref, an error of the integrator amplifier 2, an error of the step-up circuit 4, or the like. The detected air-fuel ratio is less accurate than a stoichiometric ratio which would be detected solely on the basis of an electromotive force depending on the oxygen concentration difference.
Automotive emission control systems with three-way catalytic converters are required to control the air-fuel ratio within a narrow range or window close to a stoichiometric ratio. Therefore, it is important to detect the stoichiometric ratio with high accuracy because the three-way catalytic converter can achieve a well-balanced purification of toxic pollutants with a high purifying efficiency in the vicinity of the stoichiometric ratio, as shown in FIG. 9.
The conventional linear A/F sensor of the type described above is employed in some automotive emission control systems for purifying exhaust gases with respect to a wide range of air-fuel ratios. Under certain engine operating conditions, such automotive emission control systems are required to effect a stoichiometric ratio feedback control process for keeping the air-fuel ratio within a narrow range or window. If the air-fuel ratio falls out of the window, the three-way catalytic converter would fail to purify the exhaust gases with high efficiency. To avoid such a drawback by minimizing any errors in the feedback control system in the linear A/F sensor shown in FIG. 14, the voltage drop across the resistor 5 is supplied to a current inversion detector 6 to detect the direction in which the pump current Ip flows. A stoichiometric ratio signal Vstc produced by the current inversion detector 6 is thus indicative of the direction of the pump current Ip.
More specifically, as shown in FIG. 15, the pump current Ip is positive when the air-fuel ratio is on the leaner side of the stoichiometric ratio, and negative when the air-fuel ratio is on the richer side of the stoichiometric ratio. The pump current Ip as it is thus inverted is detected by the current inversion detector 6 as the stoichiometric ratio signal Vstc which switches between high and low levels at the stoichiometric ratio. Since the stoichiometric ratio signal Vstc does not contain an error of the gain G and an error of the step-up voltage Vstp, the stoichiometric ratio can be detected with high accuracy.
However, the stoichiometric ratio signal Vstc produced by the current inversion detector 6 still contains other errors of the feedback control system, e.g., an error of the reference voltage Vref and an error of the integrator amplifier 2. Inasmuch as the components of the feedback control system are subject to aging errors, the accuracy with which the stoichiometric ratio is detected remains to be improved. In addition, the pump current Ip based on which the stoichiometric ratio is detected contains a time lag caused by the controller 31, with the result that the stoichiometric ratio feedback control process based on the pump current Ip is relatively slow in response.
Different automobile types are characterized by different proportions and quantities of exhaust gas components at the inlet of the three-way catalytic converter. Furthermore, different catalyst types have slightly different exhaust gas purification characteristics, which results in different air-fuel ratios to achieve high purifying efficiencies of three-way catalytic converters. In view of these problems, there has been a demand for a system for effecting fine adjustments of a target air-fuel ratio in order to control the purifying efficiency of a three-way catalytic converter at a high level.